This invention relates to coating compositions for the production of electrically conductive coatings on electrically nonconductive substrates, such as, e.g., glass, ceramic materials, or plastic.
xe2x80x9cConductive lacquersxe2x80x9d are used, e.g., for coatings intended for the electromagnetic shielding (EMI screening) of electronic devices such as personal computers and portable telephones. The lacquers are applied, for this purpose, to the appropriate plastic housing by spraying or immersion, and then subsequently dried in ambient air or in an oven in order to remove volatile components. Other uses for such lacquers are in the production of printed circuit boards and in the metallizing of wax cores for electroforming processes.
Compositions for these applications customarily contain fine metallic powders, as well as, optionally, non-metallic powders, which are dispersed in a binder-solvent system.
Conventional conductive lacquers ordinarily contain large amounts of volatile organic solvents such as ethylene glycol, butyl acetate, ethyl glycol acetate, etc. These solvents are, as a rule, dangerous to workers"" health and to the environment. The maximum concentration of these components in the workplace is therefore limited. Special protective devices such as outlets, suction removal systems, filter systems and solvent recovery systems must be used in the processing and drying of the lacquers, as well as when cleaning the containers and devices used. If readily combustible solvents are involved, additional measures for protection against explosions are required.
Recent attempts have been made to replace such organic solvents with water, in order to minimize the danger to the health of workers and to reduce the strain on the environment.
DE 44 31 723 C2 describes metallizing pastes for the coating of inorganic substrates by silk-screen (sieve screen) pressure, and also by spraying and immersion. The aqueous dispersions used therein are produced with rheological auxiliary agents and contain no organic binder. These compositions therefore exhibit no chemical cross-linking during drying. Accordingly, the conductive coatings produced do not have sufficient adhesive strength and resistance to wear and to moisture. Therefore, they cannot be used for shielding purposes.
U.S. Pat. No. 5,492,653 also describes coating compositions which can be processed in the spraying method. These lacquers contain silver flake powder coated with organic carboxylic acids, and also contain a significant amount (up to 8% by weight) of organic solvents. Solvents such as, e.g., diethylene glycol monobutyl ether are used therein, which solvents are also harmful from a toxic standpoint.
Surprisingly, it has been discovered that the above requirements are met by a coating composition containing one or more conductive pigments and an organic binder, as well as, if necessary, additives and auxiliary agents, in an aqueous solvent, which organic binder is a copolymer dispersible in water and based on (meth)acrylate- and silylated, unsaturated monomers. The total content of organic solvents in the composition does not exceed 0.5% by weight.
Thus, the subject matter of the invention is a coating composition, characterized as above, for producing electrically conductive coatings.
It turns out that when such partially silylated (meth)acrylate copolymers are used as a binder in conductive coating compositions with components which are otherwise customary for this purpose, but with water as solvent, conductive coatings with excellent adhesive strength, mechanical resistance and resistance to solvents can be obtained. These qualities are thought to result from a cross-linking of silanol groups which are formed from the silyl groups contained in the copolymer. Very well-bonded, conductive lacquer coatings resistant to solvents and mechanical influence are obtained by the formation of a polysiloxane network, in addition to the film formation which takes place by the flowing of the polymer resin dispersed in the composition during the drying and hardening.
The silylated copolymer to be used in accordance with the invention as a binder consists of a basic framework of acrylate units or methacrylate units and/or vinylic monomer units, some of which carry silyl groups. A suitable silyl functioning is brought about with trialkoxysilyl groups such as, in particular, trimethoxy- and triethoxysilyl groups. The latter are readily transformed by hydrolysis into silanol groups (xe2x80x94Sixe2x80x94OH) which can be united to a polysiloxane network (xe2x80x94Oxe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94Oxe2x80x94).
Typical silylated co-monomers are, e.g., methacryloxypropyl trimethoxysilane and vinyl trimethoxysilane.
The copolymer purposefully has a degree of silylation of 0.05 to 50%. Such copolymers are readily dispersible in water, during which organic (co)solvents can be eliminated.
A typical copolymer is composed, e.g., of 45% methylmethacrylate, 50% n-butylacrylate and 5% methacryloxypropyltrimethoxysilane, and has a residual monomer content of the particular monomers of less than 0.5% by weight.
The copolymer functioning as a binder is purposefully used in the conductive coating composition of the invention in the form of an aqueous dispersion. Such an aqueous polymer dispersion has a solid content of 25 to 75% by weight, preferably 25 to 50% by weight, more preferably 30 to 40% by weight. Such aqueous polymer dispersions are self-cross-linking; their minimum film formation temperature (MFT) can be adjusted as desired in accordance with the composition. Appropriate polymer dispersions are commercially available (e.g. SD 194, Degussa-Hxc3xcls AG; SANMOL SW 131, SW 135, Sanyo Chemicals). Surprisingly, the use of these special, silylated acrylate copolymer dispersions also assures a better settling behavior in comparison to products containing solvents. This results in simple processing of the lacquer during spraying, brushing or immersion.
The coating compositions in accordance with the invention contain conductive pigments as well as, optionally, other additives and/or auxiliary agents. Preferably, silver flake powders (e.g., F14, D21, D12 or D35, Degussa-Hxc3xcls AG) or copper flake powders (e.g., type MP 6100, Eckart-Werke) can be used as conductive pigments. Even metallized, inorganic, flake pigments (e.g., EMI pigment TP1029A, Cerdec AG) can be used. In addition, powders of conductive, inorganic oxides such as, e.g., fluoride-doped stannous oxide SnO2 (F) or indium/stannous oxide In2O3/SnO2 can be used.
In particular, wetting agents, defoaming agents, thixotroping agents, adhesion promoters and cross-linking agents can be used as lacquer additives. Combinations of various defoaming-agent types and wetting-agent types can also be used (e.g., Dehydran 1227, Dehydran 1293G, Dehydran 1513 or Dehydran 1620 (Henkel KGaA, Dusseldorf). Examples of thixotroping agents are pyrogenic silicas (Aerosil(copyright), Degussa-Hxc3xcls AG). Examples of adhesion promoters and cross-linking agents are epoxysilanes (e.g., silane A174, Union Carbide company).
The conductive lacquer of the invention typically has the following composition:
2.5 to 10% by weight silylated copolymer
25 to 75% by weight conductive pigment
13 to 72.5% by weight water
0 to 3% by weight lacquer additives (defoaming agents, wetting agents, thixotroping agents, adhesion promoters, cross-linking agents)
0 to 0.5% by weight organic solvent.
Since the copolymer is used in the form of a purely aqueous dispersion, and since only water is used as the solvent, the amount of volatile organic solvents in the lacquer of the invention is determined by the amount of the lacquer additives (defoaming agents, wetting agents, etc.) in the recipe. When defoaming agents with a low solvent content are used (less than 40%), the amount of organic solvent in the conductive lacquer can be adjusted to less than 0.5% by weight relative to the entire recipe. When defoaming agents on a completely aqueous basis are used (e.g., Dehydran 1620), a completely aqueous lacquer system is obtained without any amount of organic solvents.
During the processing of such products, the dangers to the health of the workers, as well as the strain on the environment, are minimized, since only very slight amounts of organic solvents are released. Also, water can be used to dilute the lacquer of the invention. Residues of the lacquer on faultily coated parts can therefore be removed with water, and the tools for working the lacquer can be cleaned with water. Since the polymer resin itself poses no great danger, the water of the aqueous cleaning solutions also poses no great problems.
The coating compositions according to the claimed invention are used, for example, as coatings for the electromagnetic shielding (EMI screening) of electronic devices such as personal computers and portable telephones. The lacquers are applied, for this purpose, to the appropriate plastic housing by spraying or immersion, and then subsequently dried in ambient air or in an oven in order to remove volatile components. The lacquer is sprayed onto the inside of a housing of a mobile telephone, or the like, the spray time being selected so that a coating thickness of 10 xcexcm is achieved after drying. The coatings obtained are resistant to wiping, scratching and moisture.
The coatings according to the claimed invention exhibit an electrical conductivity (sheet resistivity) which is in the range of 0.01 to 100 Ohms/square, preferably from 10-100 mOhm/square, more preferably from 25-75 mOhm/square, for example in the range of 35-50 mOhm/square, relative to a lacquer-coating thickness of 10 xcexcm. (The conductivity values are given as sheet resistivity values in Ohms/square, with reference to a coating thickness of 10 microns. The cited values are determined according to techniques well known in the art, according to conventional testing standards.) The amount of volatile organic solvents in such lacquers is less than 0.5% by weight relative to the total recipe. Other uses for such lacquers are in the production of printed circuit boards, and in the metallizing of wax cores for electroforming processes, where equally satisfactory results are obtained.